1. Field of the Invention
The present invention relates to a packet forwarding device capable of forwarding packets between wireless communication networks and a wireless communication system including the packet forwarding device.
2. Description of the Related Art
In recent years, wireless communication systems in which field equipments (sensors or actuators), called wireless field devices, capable of performing wireless communication are installed and communication is performed via wireless communication networks based on control signals used to control the wireless field devices or measurement signals or the like obtained with the wireless field devices are being implemented in plants, factories, or the like. As a communication standard used in such wireless communication systems, ISA100.11a which is a wireless communication standard for industrial automation developed by the International Society of Automation (ISA) can be exemplified.
A wireless communication system according to the foregoing wireless communication standard, ISA100.11a, includes a field device, a backbone router that forms a wireless communication network with the field device, and a system manager that generally controls the wireless communication performed via the wireless communication network. Under the control of the system manager, the wireless communication is performed according to Time Division Multiple Access (TDMA) using a plurality of communication channels (for example, 16 channels).
Here, to allow the foregoing wireless field device to participate in the wireless communication network, an operation of setting device information (a network parameter and security parameter), called “provisioning,” is required to be performed on the wireless field device. Methods of performing the “provisioning” are broadly classified into Over The Air (OTA) provisioning in which the device information is set by performing wireless communication in conformity with the foregoing wireless communication standard, ISA100.11a, and Out-Of-Band (OOB) provisioning in which the device information is set by performing communication by communication means (for example, infrared communication) different from the wireless communication.
In “ISA-100.11a-2009 Wireless systems for industrial automation: Process control and related applications”, 14 Provisioning, pp. 666 to 693, as a method of performing the foregoing OTA provisioning, a method of performing the provisioning using a dedicated tool (provisioning device) for provisioning and a method of performing the provisioning without using the dedicated tool are defined. Specifically, the former method is a method of setting device information by installing another wireless communication network (provisioning network) physically separated from a wireless communication network (target network) in which a wireless field device participates and performing wireless communication between a tool and the wireless field device via the provisioning network. On the other hand, the latter method is a method of constructing a logically separated provisioning network on a target network and setting device information in a field device via the provisioning network from the target network.
In a wireless communication system, however, a method of dividing a wireless communication network into a plurality of small wireless communication networks (wireless sub-networks) and managing the small wireless communication networks is generally used from the viewpoint of communication resource distribution or effective management. The divided wireless sub-networks are managed by attaching different identifiers (subnet ID). Therefore, the target network and the provisioning network described above are managed by attaching different subnet IDs.
The above-described wireless communication standard, ISA100.11a, defines the details of a structure (specifically, transmission by the data link layer of the OSI reference model) in which packets are forwarded within one wireless sub-network in which subnet IDs are the same, but does not define details of a structure in which packets are forwarded between wireless sub-networks to which different subnet IDs are attached. For this reason, packets are not forwarded between a target network and a provisioning network to which different subnet IDs are attached. In the end, the OTA provisioning in which the above-described provisioning device is not used may not be realized.
Here, when routing control is performed with the network layer located above the data link layer of the OSI reference model, forwarding of packets between wireless sub-networks with different subnet IDs can be considered to be possible. In the above-described wireless communication standard, ISA100.11a, however, a wireless communication network and a core network called a “backbone network” are merely distinguished from each other in a routing table used for routing control of the network layer. For this reason, to distinguish individual wireless sub-networks that form a wireless communication network, the routing table used for the routing control of the network layer is required to be considerably modified.
As described above, since the system manager generally controls the wireless communication performed via the wireless communication network in the wireless communication standard, ISA100.11a, the above-described routing table can be generated by a system manager and can be assigned to a field device. Accordingly, when the above-described routing table is modified, it is also necessary to modify a communication protocol used to assign wireless communication resources defined in the routing table, and thus the wireless communication standard, ISA100.11a, is required to be modified.